Natural gas coming from below the surface of the earth has contained associated gas which accompanies the production. Such associated gas contains acid gas containing CO2, that is, gas with greenhouse effects. For this reason, in a natural gas refinery plant, CO2 is separated and recovered from the natural gas, and the purified gas is delivered as pure natural gas to a process to produce final salable products.
As such a purification process, a process has been disclosed which includes: after removing water from raw natural gas, performing cryogenic separation by compressing and cooling to recover CO2-rich gas as liquefied carbonic acid gas; further compressing hydrocarbon rich gas from which CO2 is removed; and performing membrane separation with a polymer separation membrane to recover pure natural gas having a CO2 concentration of 10% by volume or less (Patent Literature 1).
In addition, a process has been also disclosed which includes: after removing water from raw natural gas, performing membrane separation with a polymer separation membrane in a first stage, followed by cryogenic separation with compressing and cooling to recover CO2-rich gas as liquefied carbonic acid gas; and further using CH4-rich gas, from which CO2 is removed, as sweep gas for membrane separation in a second stage (Patent Literature 2). In Patent Literatures 1 and 2, the CO2-rich gas permeated through the separation membrane is circulated to the cryogenic separation or the membrane separation in the preceding stage.
In order to achieve pure natural gas with required product quality (for example, a CO2 concentration of 2% by volume or less), however, an example such as the aforementioned ones requires a huge membrane area, and therefore an apparatus and a system may have structures large in scale, and accordingly consume enormous amounts of energy. For this reason, in reality, CO2 is separated by membrane separation using a membrane area smaller than necessary, and thereafter the CO2 concentration is reduced to product quality by an additional process.